package panda.lang.builder;

/**
 * <p>
 * Assists in implementing {@link Object#hashCode()} methods.
 * </p>
 *
 * <p>
 * This class enables a good <code>hashCode</code> method to be built for any class. It follows the rules laid out in
 * the book <a href="http://java.sun.com/docs/books/effective/index.html">Effective Java</a> by Joshua Bloch. Writing a
 * good <code>hashCode</code> method is actually quite difficult. This class aims to simplify the process.
 * </p>
 *
 * <p>
 * The following is the approach taken. When appending a data field, the current total is multiplied by the
 * multiplier then a relevant value
 * for that data type is added. For example, if the current hashCode is 17, and the multiplier is 37, then
 * appending the integer 45 will create a hashcode of 674, namely 17 * 37 + 45.
 * </p>
 *
 * <p>
 * All relevant fields from the object should be included in the <code>hashCode</code> method. Derived fields may be
 * excluded. In general, any field used in the <code>equals</code> method must be used in the <code>hashCode</code>
 * method.
 * </p>
 *
 * <p>
 * To use this class write code as follows:
 * </p>
 *
 * <pre>
 * public class Person {
 *   String name;
 *   int age;
 *   boolean smoker;
 *   ...
 *
 *   public int hashCode() {
 *     // you pick a hard-coded, randomly chosen, non-zero, odd number
 *     // ideally different for each class
 *     return new HashCodeBuilder(17, 37).
 *       append(name).
 *       append(age).
 *       append(smoker).
 *       toHashCode();
 *   }
 * }
 * </pre>
 *
 * <p>
 * If required, the superclass <code>hashCode()</code> can be added using {@link #appendSuper}.
 * </p>
 * 
 * @author yf.frank.wang@gmail.com
 *
 */
public class HashCodeBuilder implements Builder<Integer> {
	// -------------------------------------------------------------------------
	/**
	 * Constant to use in building the hashCode.
	 */
	private final int iConstant;

	/**
	 * Running total of the hashCode.
	 */
	private int iTotal = 0;

	/**
	 * <p>
	 * Uses two hard coded choices for the constants needed to build a <code>hashCode</code>.
	 * </p>
	 */
	public HashCodeBuilder() {
		iConstant = 37;
		iTotal = 17;
	}

	/**
	 * <p>
	 * Two randomly chosen, non-zero, odd numbers must be passed in. Ideally these should be
	 * different for each class, however this is not vital.
	 * </p>
	 * <p>
	 * Prime numbers are preferred, especially for the multiplier.
	 * </p>
	 * 
	 * @param initialNonZeroOddNumber a non-zero, odd number used as the initial value
	 * @param multiplierNonZeroOddNumber a non-zero, odd number used as the multiplier
	 * @throws IllegalArgumentException if the number is zero or even
	 */
	public HashCodeBuilder(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber) {
		if (initialNonZeroOddNumber == 0) {
			throw new IllegalArgumentException("HashCodeBuilder requires a non zero initial value");
		}
		if (initialNonZeroOddNumber % 2 == 0) {
			throw new IllegalArgumentException("HashCodeBuilder requires an odd initial value");
		}
		if (multiplierNonZeroOddNumber == 0) {
			throw new IllegalArgumentException("HashCodeBuilder requires a non zero multiplier");
		}
		if (multiplierNonZeroOddNumber % 2 == 0) {
			throw new IllegalArgumentException("HashCodeBuilder requires an odd multiplier");
		}
		iConstant = multiplierNonZeroOddNumber;
		iTotal = initialNonZeroOddNumber;
	}

	/**
	 * <p>
	 * Append a <code>hashCode</code> for a <code>boolean</code>.
	 * </p>
	 * <p>
	 * This adds <code>1</code> when true, and <code>0</code> when false to the
	 * <code>hashCode</code>.
	 * </p>
	 * <p>
	 * This is in contrast to the standard <code>java.lang.Boolean.hashCode</code> handling, which
	 * computes a <code>hashCode</code> value of <code>1231</code> for
	 * <code>java.lang.Boolean</code> instances that represent <code>true</code> or
	 * <code>1237</code> for <code>java.lang.Boolean</code> instances that represent
	 * <code>false</code>.
	 * </p>
	 * <p>
	 * This is in accordance with the <quote>Effective Java</quote> design.
	 * </p>
	 * 
	 * @param value the boolean to add to the <code>hashCode</code>
	 * @return this
	 */
	public HashCodeBuilder append(boolean value) {
		iTotal = iTotal * iConstant + (value ? 0 : 1);
		return this;
	}

	/**
	 * <p>
	 * Append a <code>hashCode</code> for a <code>boolean</code> array.
	 * </p>
	 * 
	 * @param array the array to add to the <code>hashCode</code>
	 * @return this
	 */
	public HashCodeBuilder append(boolean[] array) {
		if (array == null) {
			iTotal = iTotal * iConstant;
		}
		else {
			for (boolean element : array) {
				append(element);
			}
		}
		return this;
	}

	// -------------------------------------------------------------------------

	/**
	 * <p>
	 * Append a <code>hashCode</code> for a <code>byte</code>.
	 * </p>
	 * 
	 * @param value the byte to add to the <code>hashCode</code>
	 * @return this
	 */
	public HashCodeBuilder append(byte value) {
		iTotal = iTotal * iConstant + value;
		return this;
	}

	// -------------------------------------------------------------------------

	/**
	 * <p>
	 * Append a <code>hashCode</code> for a <code>byte</code> array.
	 * </p>
	 * 
	 * @param array the array to add to the <code>hashCode</code>
	 * @return this
	 */
	public HashCodeBuilder append(byte[] array) {
		if (array == null) {
			iTotal = iTotal * iConstant;
		}
		else {
			for (byte element : array) {
				append(element);
			}
		}
		return this;
	}

	/**
	 * <p>
	 * Append a <code>hashCode</code> for a <code>char</code>.
	 * </p>
	 * 
	 * @param value the char to add to the <code>hashCode</code>
	 * @return this
	 */
	public HashCodeBuilder append(char value) {
		iTotal = iTotal * iConstant + value;
		return this;
	}

	/**
	 * <p>
	 * Append a <code>hashCode</code> for a <code>char</code> array.
	 * </p>
	 * 
	 * @param array the array to add to the <code>hashCode</code>
	 * @return this
	 */
	public HashCodeBuilder append(char[] array) {
		if (array == null) {
			iTotal = iTotal * iConstant;
		}
		else {
			for (char element : array) {
				append(element);
			}
		}
		return this;
	}

	/**
	 * <p>
	 * Append a <code>hashCode</code> for a <code>double</code>.
	 * </p>
	 * 
	 * @param value the double to add to the <code>hashCode</code>
	 * @return this
	 */
	public HashCodeBuilder append(double value) {
		return append(Double.doubleToLongBits(value));
	}

	/**
	 * <p>
	 * Append a <code>hashCode</code> for a <code>double</code> array.
	 * </p>
	 * 
	 * @param array the array to add to the <code>hashCode</code>
	 * @return this
	 */
	public HashCodeBuilder append(double[] array) {
		if (array == null) {
			iTotal = iTotal * iConstant;
		}
		else {
			for (double element : array) {
				append(element);
			}
		}
		return this;
	}

	/**
	 * <p>
	 * Append a <code>hashCode</code> for a <code>float</code>.
	 * </p>
	 * 
	 * @param value the float to add to the <code>hashCode</code>
	 * @return this
	 */
	public HashCodeBuilder append(float value) {
		iTotal = iTotal * iConstant + Float.floatToIntBits(value);
		return this;
	}

	/**
	 * <p>
	 * Append a <code>hashCode</code> for a <code>float</code> array.
	 * </p>
	 * 
	 * @param array the array to add to the <code>hashCode</code>
	 * @return this
	 */
	public HashCodeBuilder append(float[] array) {
		if (array == null) {
			iTotal = iTotal * iConstant;
		}
		else {
			for (float element : array) {
				append(element);
			}
		}
		return this;
	}

	/**
	 * <p>
	 * Append a <code>hashCode</code> for an <code>int</code>.
	 * </p>
	 * 
	 * @param value the int to add to the <code>hashCode</code>
	 * @return this
	 */
	public HashCodeBuilder append(int value) {
		iTotal = iTotal * iConstant + value;
		return this;
	}

	/**
	 * <p>
	 * Append a <code>hashCode</code> for an <code>int</code> array.
	 * </p>
	 * 
	 * @param array the array to add to the <code>hashCode</code>
	 * @return this
	 */
	public HashCodeBuilder append(int[] array) {
		if (array == null) {
			iTotal = iTotal * iConstant;
		}
		else {
			for (int element : array) {
				append(element);
			}
		}
		return this;
	}

	/**
	 * <p>
	 * Append a <code>hashCode</code> for a <code>long</code>.
	 * </p>
	 * 
	 * @param value the long to add to the <code>hashCode</code>
	 * @return this
	 */
	// NOTE: This method uses >> and not >>> as Effective Java and
	// Long.hashCode do. Ideally we should switch to >>> at
	// some stage. There are backwards compat issues, so
	// that will have to wait for the time being. cf LANG-342.
	public HashCodeBuilder append(long value) {
		iTotal = iTotal * iConstant + ((int)(value ^ (value >> 32)));
		return this;
	}

	/**
	 * <p>
	 * Append a <code>hashCode</code> for a <code>long</code> array.
	 * </p>
	 * 
	 * @param array the array to add to the <code>hashCode</code>
	 * @return this
	 */
	public HashCodeBuilder append(long[] array) {
		if (array == null) {
			iTotal = iTotal * iConstant;
		}
		else {
			for (long element : array) {
				append(element);
			}
		}
		return this;
	}

	/**
	 * <p>
	 * Append a <code>hashCode</code> for an <code>Object</code>.
	 * </p>
	 * 
	 * @param object the Object to add to the <code>hashCode</code>
	 * @return this
	 */
	public HashCodeBuilder append(Object object) {
		if (object == null) {
			iTotal = iTotal * iConstant;

		}
		else {
			if (object.getClass().isArray()) {
				// 'Switch' on type of array, to dispatch to the correct handler
				// This handles multi dimensional arrays
				if (object instanceof long[]) {
					append((long[])object);
				}
				else if (object instanceof int[]) {
					append((int[])object);
				}
				else if (object instanceof short[]) {
					append((short[])object);
				}
				else if (object instanceof char[]) {
					append((char[])object);
				}
				else if (object instanceof byte[]) {
					append((byte[])object);
				}
				else if (object instanceof double[]) {
					append((double[])object);
				}
				else if (object instanceof float[]) {
					append((float[])object);
				}
				else if (object instanceof boolean[]) {
					append((boolean[])object);
				}
				else {
					// Not an array of primitives
					append((Object[])object);
				}
			}
			else {
				iTotal = iTotal * iConstant + object.hashCode();
			}
		}
		return this;
	}

	/**
	 * <p>
	 * Append a <code>hashCode</code> for an <code>Object</code> array.
	 * </p>
	 * 
	 * @param array the array to add to the <code>hashCode</code>
	 * @return this
	 */
	public HashCodeBuilder append(Object[] array) {
		if (array == null) {
			iTotal = iTotal * iConstant;
		}
		else {
			for (Object element : array) {
				append(element);
			}
		}
		return this;
	}

	/**
	 * <p>
	 * Append a <code>hashCode</code> for a <code>short</code>.
	 * </p>
	 * 
	 * @param value the short to add to the <code>hashCode</code>
	 * @return this
	 */
	public HashCodeBuilder append(short value) {
		iTotal = iTotal * iConstant + value;
		return this;
	}

	/**
	 * <p>
	 * Append a <code>hashCode</code> for a <code>short</code> array.
	 * </p>
	 * 
	 * @param array the array to add to the <code>hashCode</code>
	 * @return this
	 */
	public HashCodeBuilder append(short[] array) {
		if (array == null) {
			iTotal = iTotal * iConstant;
		}
		else {
			for (short element : array) {
				append(element);
			}
		}
		return this;
	}

	/**
	 * <p>
	 * Adds the result of super.hashCode() to this builder.
	 * </p>
	 * 
	 * @param superHashCode the result of calling <code>super.hashCode()</code>
	 * @return this HashCodeBuilder, used to chain calls.
	 */
	public HashCodeBuilder appendSuper(int superHashCode) {
		iTotal = iTotal * iConstant + superHashCode;
		return this;
	}

	/**
	 * <p>
	 * Return the computed <code>hashCode</code>.
	 * </p>
	 * 
	 * @return <code>hashCode</code> based on the fields appended
	 */
	public int toHashCode() {
		return iTotal;
	}

	/**
	 * Returns the computed <code>hashCode</code>.
	 * 
	 * @return <code>hashCode</code> based on the fields appended
	 */
	public Integer build() {
		return Integer.valueOf(toHashCode());
	}

	/**
	 * <p>
	 * The computed <code>hashCode</code> from toHashCode() is returned due to the likelihood of
	 * bugs in mis-calling toHashCode() and the unlikeliness of it mattering what the hashCode for
	 * HashCodeBuilder itself is.
	 * </p>
	 * 
	 * @return <code>hashCode</code> based on the fields appended
	 */
	@Override
	public int hashCode() {
		return toHashCode();
	}
}
